University of Groningen

Production of plasticized thermoplastic starch by spray dryingNiazi, Muhammad Bilal Khan

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Chapter 1

Introduction:

Production of plasticized thermoplastic starch by spray drying

Scope of the Thesis

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Abstract

As introduction to this thesis, this chapter provides a general overview on starch properties,

chemical and physical modification techniques and the applications of starch with emphasis on

thermoplastic starch (TPS). The use of plasticizers to process starch as thermoplastic polymer and

the formulation of starch with other polymers are highlighted. Traditional plastic processing

techniques used for TPS will be discussed and the use of “spray-drying” as a method to produce

“dry” plasticized starch will be introduced. Finally, an outline of this thesis is presented.

Content and Aim of the Thesis

The extraction of starch from agricultural products, its conversion into technical and consumer

products, and the application of these products may be considered as one of the oldest branches of

the chemical industry. At present, the global production of starches and starch products is reported

to be around 70 million tons/year and commercial products containing starch or starch derivatives

are found in almost every chemical product sector [1].

Over the years a lot of know-how has been collected and extensive reviews covering technical

and scientific aspects of starches obtained from different resources have been issued. Reviews [2-4]

and books [5-7] cover the macro-level elements related the extraction, processing and modification

of the different starches at commercial scale, the meso-level ones connected to product performance

in applications, and also the micro-level aspects related to molecular structure and biosynthesis.

However, well developed “resource-processing-product performance relationships” to support the

development of new starch applications are still absent, with the result that new product

development continues to be done on experience-based heuristics combined with trial-and-error

experimentation. The work described in this thesis aims to contribute to the establishment of

“resource-processing-product performance relationships” to support the development of novel

product applications based on thermoplastic starch (TPS). TPS is chosen because the development

of starch-based applications such as biodegradable packaging attracts a lot of research.

As an introduction for the topics discussed in this thesis, this chapter presents a summary of

required background such as basic starch characteristics, pathways to modify starches and the basic

processing technologies. The chapter closes with an overview of the thesis covering the contents of

the different chapters.

Starch

Starch is a natural glucose-based polymer that generally is considered as a potential candidate

for future biodegradable polymer products. Starch, as extracted from various plant tissues, is

obtained in the form of granules with typical particle sizes between 1-100 microns [8] and the shape

and size of the granules depend on the resource [9, 10]. These granules are present in the

chloroplasts of green leaves and in the amyloplasts of storage organs such as seeds and tubers [11].

Granules are in fact insoluble in cold water but swell, and form a gel if the outer membrane has

been broken by grinding. On the other hand, if a granule is treated in warm water, a soluble portion

of the starch diffuses through the granule wall and the remainder of the granules swells to such an

extent that they burst [10].

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Starch is important for human nutrition. However, its nutritional value depends on its primary

composition, physical structure and subsequent processing [9]. The most important sources for

starches are corn, potato and cassava [12]. In 2008, the worldwide production of starch was

estimated to be around 66 million tons [13]. The USA is at the top in the production of starch,

Europe and Asia follow as second and third, respectively [14].

Starch is a versatile and useful polymer not only because it is low cost and obtained from

natural resources, but also because its physicochemical properties can be altered through chemical

or enzymatic modification. Thus, starches receive extensive attention in food research [15], as, for

example, in Western Europe approximately two thirds of the production is used in the food and

beverage industry. It is directly applied as thickener in sauces, custards and desserts and can, after

enzymatic hydrolysis, be used as sweetener in drinks and confectionery. Starch is also important for

non-food purposes as the remaining one third of total starch is used in processes like sizing of paper

and board, and as adhesives in the paper, packaging and textile industries. Polyols, cyclodextrins,

fructose, antibiotics and related compounds are obtained from starch via various fermentation

processes [13].

Chemical and physical properties of starch

Starch mainly consists of two different polysaccharides: amylose and amylopectin, but the ratio

of these two strongly varies depending on the starch source. Small amounts of lipids and proteins

are also present in the starch granules [2].

Amylose is a predominantly linear chain of D-glucose units linked together by α-1, -4 bonds

(Figure 1). The length of the amylose chain is not the same for every source; it varies among

different plant species but usually ranges between 102 to 10

4 glucose units [16].

Amylopectin consists of short chains of, on the average, 20 to 30 α-1,4 linked D-glucose units

which are branched by α-1,6 bonds (Figure 1). Usually it ranges to 104 to 10

5 glucose units [16].

Figure 1 Linear and branched starch polymers (reproduced with permission of the publisher) [7].

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The relative percentage of amylose and amylopectin in most common types of cereal

endosperm starches ranges between 72 and 82 % amylopectin, and 18 and 28 % amylose. However,

some mutant genotypes of maize (Zeamays), barley (Hordeum vulgare), and rice (Oryza sativa)

contain as much as 70%, amylose whereas other genotypes, called waxy, contain less than 1%

(maize, barley, rice, sorghum) [17].

Starch is insoluble in cold water, but is very hygroscopic and binds water reversibly. Heating

starch in water leads to loss of hydrogen bonding in the interior of the starch granule and the starch

will start to gelatinize. The starch granules will rapidly swell to many times its original volume. The

gelatinization temperature range can be defined as the temperature at which granular swelling

begins until the temperature at which nearly 100% of the granules are gelatinized [18].

Physically, most native starches having a crystallinity of about 20-40%. They are classified as

semi crystalline [19]. The amorphous regions consist of amylose and the branching points of

amylopectin. The main crystalline components in granular starch are short-branched chains in the

amylopectin. The crystalline regions are present in the form of double helices with a length of ~ 5

nm.

Modification of starch

Apart from the immediate use as food, native starch has a limited range of applications. This is

due to its unfavorable properties like uncontrolled and high viscosity, its tendency to retrogradate,

the insolubility in cold water, and the pronounced brittleness of processed materials [3, 15, 20]. As a

consequence, native starches are often modified chemically or physically in order to change and

improve product properties. Modification alters the starch structure and affects the hydrogen

bonding in a controllable manner to enhance and extend its application. It is remarkable that many

of the modifications can be executed at the molecular level, without significantly changing the

superficial appearance of the granule [7]. Well-known chemical and biochemical modifications are

described below and represented schematically in Figure 2.

Cross-linking

In the starch industry, cross-linking is one of the most important chemical modifications. The

weak hydrogen bonding between starch chains is replaced by stronger, more permanent, covalent

bonds. The majority of the glucose units in starch contain two secondary and one primary hydroxyl

group, which can react easily with a wide range of compounds such as acid anhydrides, organic

chloro-compounds, aldehydes, epoxy, and ethylenic compounds. Covalent bonding or cross-linking

is more permanent in nature, so only a small degree is required to produce beneficial effects:

typically one cross-link per 100-3000 anhydroglucose units of starch. Starch becomes more resistant

to gelatinization as the number of cross-links increases. Consequently, solubility of the cross-linked

starch derivatives in water is reduced and results in a higher freeze thaw stability, and a higher acid,

heat, and shear stability. Also a lower tendency for retrogradation and gelatinization, and higher

viscosity over their parent native starches [3, 20, 21] has been reported. The cross-linked starches

have found many useful applications, especially in the food, paper, textile, and adhesive industry [3,

22].

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Figure 2 Chemical and biochemical modifications of starch (reproduced with permission of the publisher) [7].

Stabilization and Lipophilic Substitution

Stabilization is used in conjunction with cross-linking. It is done to prevent retrogradation and

to increase the tolerance of starch to temperature fluctuations such as freeze-thaw cycles.

Consequently, it helps to improve the shelf life of starch products. In this modification bulky

groups, such as octenylsuccinate [7] , are substituted onto the starch in order to take up space and

prevent dispersed, linear fragments to re-align and retrograde. The number and nature of the

substituted group determines the effectiveness of stabilization. The number of substituents per 100

anhydroglucose units represents the degree of substitution (DS). Starch-starch interactions in the

granule are weakened with an increase in DS level and, consequently, hydration and gelatinization

is achievable at lower temperatures by cooking [7].

Native starch is hydrophilic due to the presence of polyhydroxyl groups in the polymer chain,

and does not possess sufficient affinity for hydrophobic substances such as oil to be an effective

emulsifier. However, through chemical modification the lipophilic functionality of starch may be

improved [23]. The natural tendency of starch to interact with water can be transformed into

hydrophilic-hydrophobic duality upon lipophilic substitution. This is particularly helpful to stabilize

interactions between materials such as oil and water. To perform such modification, hydrophilic

starch must be provided with a long hydrophobic, i.e. lipophilic, hydrocarbon chain [7]. Starch

reaction with 2-(2’-octeny1)-succinic anhydride is one of the examples of such modification. It

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results in the production of 2-(2’-octeny1)-succinic acid (OSA) modified starch. OSA binds

covalently through esterification on starch, and provides starch with both hydrophilic and lipophilic

functionalities due to the presence of carboxyl and octenyl groups, respectively. OSA modified

starches are useful in food and pharmaceutical applications due to presence of dual functionalities in

which the stabilization of oils exposed to an aqueous environment is desirable without the use of

proteins and/or gums for medical or technological reasons [7, 23].

Esterification

Starch esterification is a well-known chemical modification in which some hydroxyl groups

have been replaced by ester groups. The objective is to improve the gelatinization temperature and

thermal stability, or to reduce the tendency for retrogradation [6, 20]. The synthesis of starch esters

was already introduced 150 years ago, starting with the development of acetylated starch using

acetic anhydride as the acetylating agent [24]. Different varieties of starch esters are available such

as starch formates, starch nitrates, starch sulfates, starch phosphates, fatty acid starches, starch

alkenyl succinates and starch xanthates [3, 24]. Acetylated starch is still an important commercial

product and used in many applications. Relatively low degrees of substituted (DS) products are

available on commercial level (DS 0.01-0.2). Starch acetates are divided into three different types

depending on DS value [24]. Commercially available low DS products (0.01-0.2) are used as food

additives and in the textile industry. Medium DS starch acetates (0.3-1) are soluble in water but high

substituted starch acetate (DS of 2-3) are in organic solvents. Starch esterification with acetate

groups makes the esters more hydrophobic than native starch. High DS starch acetate is useful to

cast films using organic solvents. Starch acetylation is a controlled process and allows producing

polymers with a range of hydrophobicities [25]. High DS starch acetates are thermoplastic materials

and used as biodegradable plastics [26].

Starch esters can be made by several synthetic methods. Water is used as solvent on

commercial scale with acetic anhydride as reactant and an alkaline base such as sodium hydroxide,

sodium acetate, or sodium hydroxide as the catalyst [27, 28].

Fatty acid starch esters have received considerable attention in the last decade. These

hydrophobic starch derivatives have a high application potential not only in the food but also in

non-food industry (e.g. as reactive compound in polyurethane resins) [29] and as substitutes for oil-

based polymers especially in the packaging industries [30, 31]. In the food industry, starch esters are

used as binders, thickeners, and fillers. In non-alcoholic beverages it is used for the stabilization of

flavor, and for frozen fruits, pies and pot pies it is used to maintain the stability at low temperature

storage [6]. Starch succinates have high thickening power, freeze-thaw stability, and a low

gelatinization temperature. In pharmaceutical applications [20] it is effectively used as tablet

disintegrant.

Conversions

Chain-cleavage reactions of starch are collectively termed as conversion.

Acid Hydrolysis

Acid hydrolysis is an important modification to improve the physicochemical properties of

starch. The acid attacks the glycosidic oxygen atom and hydrolyses the glycosidic linkage.

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Consequently, the physicochemical properties of starch changes without destroying its granular

structure [32]. These starches can be used at higher solid concentrations to achieve quick gelling

and to provide gum or jelly with controlled texture and flexible properties [33]. It is also

extensively used in textile and paper industries [34].

Oxidation

Starch oxidation is already applied since early 1800. A number of oxidizing agents has been

used, for instance, hypochlorite, hydrogen peroxide, periodate, permanganate, dichromate, per-

sulfate, and chlorate [35]. Oxidized starches are used in textiles, laundry finishing, building

materials and food industries [32]. The main application of oxidized starch is in paper and textile

industries, where it is used to improve the strength and printability of paper, and as warp sizing,

respectively. Due to their low viscosity, high stability, clarity, and good binding properties, oxidized

starches are widely used in the food industry, in applications such as food coating, sealing agents in

confectionery or as emulsifiers. Hypochlorite oxidation is the most common method for the

production of oxidized starches at industrial scale [6, 32]. During oxidation of starch with

hypochlorite, cleavage of polymer chains and oxidation of hydroxyl groups to carbonyl and

carboxyl groups takes place. Oxidation occurs randomly at primary hydroxyls (C-6), secondary

hydroxyls (C-2, C-3, and C-4), at reducing end-groups, and at glycol groups (cleavage of C-2─C-3

bonds). The number of carboxyl and carbonyl groups on oxidized starch indicates the level of

oxidation, which takes place primarily at the hydroxyl groups at C-2, C-3, and C-6 positions [20].

Dextrinisation

Dextrinisation, also known as pyro-conversion, is the process involving the browning of starch

foods when subjected to dry heat. It is defined as the breakdown of starch into dextrin’s. It refers to

two pathways of structural modification of starch. The first is partial de-polymerization achieved

through addition of water. It is usually done by dry roasting the starch either alone, using the

moisture contents present in the starch, or in the presence of catalytic quantities of acid. As a result,

polymer fractions of varying chain length (low conversion) are obtained. The second path involves

the re-polymerization in a branched manner (high conversion) and delivers products called dextrins

or pyrodextrins. Depending on their properties, e.g. range of viscosity, cold-water solubility, color,

reducing sugar contents and stability, they are classified as white dextrins, yellow dextrins or british

gums [6].

Enzymatic hydrolysis

Enzymatic hydrolysis is a biochemical modification reaction. Selective enzymes are used in the

hydrolysis to alter the starch structure and to achieve desired functionality. A range of chain lengths

corresponding to glucose (dextrose), maltose, oligosaccharides and polysaccharides can be

produced depending on the extent of hydrolysis. α-Amylase, β-amylase, glucoamylase, pullulanase,

and isoamylase are the most common enzymes used for starch modification. These enzymes have

been isolated from fungi, yeasts, bacteria, and plants [6]. The exact composition can be determined

by dextrose equivalent (DE), which is used to define the reducing power of the product and is a

measure of the degree of starch breakdown. A DE of 0 defines native starch. Dextrose equivalent

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(DE) is used to define the reducing power of the product and is a measure of the degree of starch

breakdown

Starch liquefaction and saccharification are different approaches applied in enzyme hydrolysis.

The liquefaction process involves the conversion of a concentrated suspension of starch granules

into a solution of low viscosity by α-amylase. Saccharification involves enzymes such as

glucoamylase (amyloglucosidase) to convert starch or intermediate starch hydrolysis products to D-

glucose [6].

Pregelatinisation

Pregelatinisation is a physical modification. Certain starches need cooking to develop their

required properties and the process of pregelatinisation is designed to remove the necessity for

cooking. To achieve a versatile range of cold thickening starches, pregelatinisation is applied to

native or modified cook-up starches [7]. The starch is pre-cooked by using one of the following

processes [6]:

Drum drying (starch suspension or starch pastes)

Extrusion ( semi-dry starch)

Spray drying of starch suspension

Solvent-based processing

During a spray cooking process, a starch slurry enters in a heating chamber through a special

nozzle and is atomized. At the same time, hot steam is injected through a second or the same nozzle

into the chamber to cook out the starch [36]. This method produces uniform gelatinized starch with

minimum shear and heat damage. Spray drying is gaining popularity and offers properties that are

closer to base cook-up starch [6].

Drum drying produces a cooked starch sheet from starch slurry. The starch is ground after

drying to a desired particle size [37]. Slightly lower viscosities are obtained than their base starch

when the drum-dried flakes are ground to the desired particle size.

Solvent-based methods produce cold water swellable starches. For example, when 20%w starch

in aqueous alcohol (20 to30%w water) is heated to 160 to 175 ºC for 2 to 5 min granular integrity is

maintained, but birefringence is lost [38].

Pregelatinized starches are mainly used as thickening agent for pie fillings, puddings, sauces,

and baby foods.

Thermoplastic starch (TPS)

Native starch is unsuitable for many applications due to its brittle and hydrophilic nature.

Moreover, the melting point of starch is higher than the thermal decomposition temperature, which

results in poor thermal process-ability [39]. Starch can be processed into a thermoplastic material

only in the presence of plasticizers (water, glycerol, glycol, sorbitol, etc.) in combination with

temperature and shear [39, 40]. During the last decade, numerous studies have been done on

thermoplastic starch. Recently, much interest has been shown to produce biodegradable plastic

items [39, 41, 42], such as trash bags, shopping bags, and dinner utensils, planting pots, diapers, and

tampons.

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Figure 3 Schematic representation of the phase transitions of starch during thermal processing and aging

(reproduced with permission of the publisher) [2, 20].

The thermal processing of starch-based polymers involves multiple chemical and physical

reactions, e.g. water diffusion, granule expansion, gelatinization, decomposition, melting and

crystallization. Gelatinization is the most important phase transition, it involves all these processes

and thermoplastic conversion of starch is based on it [39]. Figure 3 shows a schematic

representation of the phase transitions of starch during gelatinization and retrogradation [43].

Heating in water destroys the crystalline structure of starch granules and forms amorphous

amylopectin (Figure 3 a and b). Gelatinized amorphous amylopectin still contains small quantities

of un-gelatinized amylopectin. Nevertheless gelatinization destroys the double-helical crystalline

structure formed by the short-branched chains in amylopectin. It has been noticed that these short-

branched chains form gel-balls [43], holding chains largely from the same sub-main chain (Figure

3b). Extrusion forms V-type single helix crystals (Figure 3c), having higher moduli and yield

stresses for amylose-rich materials. The crystallinity of the V-type crystals increases with time

(Figure 3d) [2]. Gelatinization can be done under shear and shearless conditions. In shearless

gelatinization, the process mainly depends on the water content and temperature. Excess of water is

required for full gelatinization of starch under shearless conditions, which Wang et al. [44] have

defined as > 63%. In shear conditions, gelatinization can be achieved at low moisture contents,

since the starch granules are physically torn apart by the shear forces, allowing faster transfer of

water into the interior molecules [45]. Extrusion of starch is one of the examples of gelatinization

under shear. In extrusion, loss of crystallinity is not caused by water penetration, but by the

mechanical disruption of molecular bonds due to the intense shear fields within the extruder [46].

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Furthermore, the starch decomposition temperature is higher than its pregelatinization melting

temperature and consequently, starch has a high glass transition temperature (Tg) [39]. Due to free

volume relaxation and retrogradation, brittleness also increases with time. To overcome these

problems and to make starch more flexible and easy to use, plasticizers are blended with starch [39,

47]. Water plays two roles in the starch gelatinization: it breaks the structure of the native granule

and acts as a plasticizer [40]. However, TPS containing only water has little value in practical

applications because of poor mechanical properties. A second plasticizer is necessary besides water,

such as poyols, urea, formamide, acetamide, and citric acid [2]. All have been evaluated as

additional plasticizers that will allow melting phases at a temperature lower than that of starch

degradation and they make starch flexible [2, 40]. During starch processing, the combination of

shear, temperature and plasticization allows the preparation of molten thermoplastic material. The

properties of plasticized starches depend very much on moisture contents. Relative humidity

changes the material properties, through a sorption-desorption mechanism [40]. Even when

moisture and temperature are controlled, the properties of the material remain dynamic, changing

into lower elongation at break and higher rigidness. So in order to improve the moisture

impermeability of starch products, starch is formulated with renewable, natural or synthetic

polymers [2, 39, 40].

Thermoplastic starch/Polymer blends

Research and development of biodegradable plastics were stimulated due to rising oil prices

and further augmented by environmental concerns. The objective of intensive academic and

industrial research was to develop plastics by using renewable, biodegradable resources, that fulfill

market needs [41]. Pure starch is highly brittle, and has poor mechanical properties and stability; it

is often reinforced with fibers or blended with synthetic polymers [2].

Blending is an important process for the modification of polymer properties. It is economical

and usually does not need special equipment and techniques. From a technological point of view, a

compatible blend is attained when the preferred properties are improved as a result of mixing two or

more polymers. TPS is blended primarily for two reasons [7]; 1) to improve water resistance and

mechanical performance, 2) to use TPS as modifier to make polymer blends biodegradable and

economical. Since the 1970s, starch blends with polar polymers having hydroxyl groups, such as

poly(vinyl alcohol) (PVA), copolymers of ethylene and partially hydrolyzed vinyl acetate (EVA)

were prepared [48-50]. Due to the hydrophilic nature of these TPS blends, water was used as the

plasticizer. Starch blends can be separated into two main categories according to [51];

The source and biodegradation properties of the polymer to be blended with starch

The processing technique for its preparation

As for the first category, the sources can be obtained directly from biodegradable biopolymers

or can be synthetic polymers from either oil or renewable resources, not depending on their

structure but biodegradable.

As for the second category, starch blends are prepared by two main processing techniques, i.e.

melting and solution/dispersion. In melt processing, plasticization of starch granules is done in an

extruder or a batch mixer to prepare the starch blend. Alternatively, two extruders are used, first a

single screw extruder is used to gelatinize the starch which is fed into a twin screw extruder where

other components blend with gelatinize starch [51, 52]. In solution or dispersion processing, casting

Introduction and Scope of the thesis

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is frequently used to finish the product. Starch/poly(ethylene-co-acrylic acid) (EAA) cast films were

the first commercial materials produced by solution or dispersion technique [48, 49].

Solution/dispersion blending is an exciting route for the production of these mixed materials, as in

case of starch blends, many natural polymers and biodegradable synthetic polymers are soluble or

dispersible in water. In some cases, when the polymer decomposes before melting, melting is not

possible. For example using chitosan, solution blending is the only viable alternative [5].

Starch can be the continuous or the dispersed phase in TPS blends, depending on the

starch/second-polymer ratio and on the processing conditions. As a consequence, the properties of

the blend are determined by the interfacial interactions between starch and the other components.

Numerous methods and polymers (Table 1) have been examined to improve the compatibility

within these blends [5]:

1) The use of polymers with polar groups, mainly capable to form hydrogen bonds (e.g. PVA,

EAA, EVOH and natural polymers like cellulose and its derivatives, gelatin and zein [4, 39, 53,

54].

2) The use of polymer mixtures, with one of them acting as a compatibilizer between starch and

less hydrophilic components (e.g. PVA in TPS/polyethylene blends or a low molecular weight

polymer like poly(ethylene glycol) in TPS/PLA blends) [4, 39, 54, 55].

3) The use of reactive compatibilizers, to obtain a better interface by polymer-polymer chemical

interlinking (e.g. methylenediphenyl diisocyanate (MDI), pyromellitic anhydride or glycidyl

methacrylate [4, 39, 56-58].

4) The development of complexes between starch and other polymers (e.g. V-type complexes)

[54, 59].

Table 1

Most common polymers used in blends with thermoplastic starch [5].

Polymer Reference

Poly(vinyl acetate) PVAc [49, 60]

Poly(methacrylic acid-co-methyl methacrylate) MAA/MMA [61]

Poly(vinyl alcohol) PVA [55, 62]

Poly(acrylic acid) PAA [59, 63]

Poly(ethylene-co-acrylic acid) EAA [48, 49]

Poly(ethylene-co- vinyl alcohol) EVOH [53, 54, 57]

-caprolactone) PCL [54, 57, 64, 65]

Poly(ethylene-vinyl acetate) [66]

Polyethylene [51, 52, 58, 66, 67]

Poly(ester-urethanes) [68]

Poly(D,L-Lactic acid) PLA [56, 69, 70]

Poly(3-hydroxybutyrate) PHB [4]

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) PHBV [71]

Poly(butylene succinate adipate) PBSA [72]

Polyesteramide PEA [64, 73]

Zein [67]

Lignin [74]

Cellulose and its derivatives [57]

Natural rubber [63, 75, 76]

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Blend with synthetic polymers

One of the main disadvantages of biodegradable polymers from renewable sources is their

dominant hydrophilic character, fast degradation, and, in some cases, unsatisfactory mechanical

properties, particularly in wet environments. In principle, the properties of natural polymers can be

significantly improved by blending with synthetic polymers [41, 47, 77].

In the 1970s, many efforts have been made to produce starch-based polymers for preserving the

petrochemical resources, decreasing environmental impact and searching further applications.

Numerous blends of starch with various poly-olefins were developed [41, 77]. The main purpose of

using starch granules in most of these cases was to increase the surface area available for attack by

microorganisms. However, these blends were only partially biodegradable, and thus the advantage

of using a biodegradable polysaccharide was lost. This is not acceptable from an ecological point of

view [77].

Blend with biodegradable polymers

To create entirely biodegradable starch-based composites, blends have been prepared with

biodegradable polymers. Starch is usually blended with components such as aliphatic polyesters,

polyvinyl alcohol (PVA) and biopolymers. The common used polyesters are poly(β-

hydroxyalkanoates) (PHA), poly-lactide (PLA) or poly(ε-caprolactone) (PCL). The goal of these

blends is to create a completely biodegradable product at low cost while maintaining other

properties at an acceptable level [77].

Plasticizer classification

Plasticizers are non-volatile low molecular weight compounds that are extensively used as

additives in polymer industries [78]. The main role is to lower the glass transition temperature (Tg)

to improve the flexibility and processibility of polymers. A plasticizer is defined as “a substance or

material incorporated into a material (usually a plastic or elastomer) to increase its flexibility,

workability, or distensibility” by IUPAC (International Union of Pure and Applied Chemistry) [47].

Plasticizers can be classified into different groups depending on their function and

characteristics. In polymer science, they can be either defined as internal or external. External

plasticizers interact with polymer chains, but are not chemically bonded to them by primary bonds,

and can, therefore, be lost by evaporation, migration, or extraction. Internal plasticizers become part

of the polymer, either are co-polymerized into the polymer structure or reacted with the original

polymer [79].

Plasticizers can also be divided into primary and secondary types [80]. For a primary

plasticizer, the polymer is soluble in the plasticizer at a high concentration of polymer. Plasticizer

should gel the polymer quickly and should not exude from the plasticized material within the

normal processing temperature range. Secondary plasticizers have low compatibility with the

polymer and low gelation capacity. Secondary ones are used to lower the cost and improve product

properties and are typically blended with primary plasticizers [81].

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Plasticizers for TPS blends

The decomposition temperature of starch is higher than its melting temperature, so it is not

possible to thermally process starch-based blends without a plasticizer or gelatinization agent [82,

83]. Many plasticizers and additives were assessed and developed to gelatinize starch during

thermal processing. The preparation/processing conditions and the mechanical and thermal

properties of the final material are determined by the type and quantity of plasticizer employed [5].

One of the most used plasticizers in the thermal processing of starch-based polymers is water [2, 47,

84]. However, TPS containing only water has poor mechanical properties, especially brittleness,

resulting from fast retrogradation and decreases its practical applications [85, 86].

Various other plasticizers were evaluated to improve the processing properties and product

performance of TPS blends e.g. glycerol [87-90], urea [91-94], formamide [92-94], sorbitol [95,

96], citric acid [97], glycol [98-101], and amino acids [102, 103]. The most common plasticizer

used after water is glycerol due to its high boiling point, availability, and low cost [41]. The

plasticizer- starch interaction can be very specific. The plasticizer interacts through hydrogen

bonding with starch chains in a wide temperature range. The interaction increases at higher

temperatures, probably due to H-bond formation. As a consequence, the material behaves like a

rubber, with a rise in matrix mobility and a decrease in viscosity [47, 104]. Plasticizers containing

amide groups were tested for TPS plasticization using glycerol as reference. Amide groups were

effective to inhibit starch retrogradation. The mechanical properties and retrogradation of TPS

blends mainly depends on the hydrogen bond-forming ability between plasticizers and starch

molecules. This ability increases in the following order: urea > formamide > acetamide > polyol

[93]. The moisture affinity of TPS blends depend on the plasticizer concentration and hydrophilic

nature [105] e.g. during storage glycerol containing films absorb more water and at a higher rate,

compared to sorbitol containing films.

Techniques used to process starch-based materials

There are various techniques that can be used for the production of bio-plastics from starch.

Most common plastic processing techniques are extrusion, casting, injection molding, and

compression molding [4]. All are similar to those extensively used in the processing of traditional

petroleum-based plastics. However, processing of starch is more complicated and difficult to control

than for conventional polymers. This is due to its unsatisfactory processing properties e.g. phase

transitions, high viscosity, water evaporation, fast retrogradation, etc. However, many of these

challenges can be overcome with proper formulations and suitable processing conditions. The

following degrees of freedom are available to the formulator [2]:

Plasticizers.

Lubricants.

Modified starches e.g. carboxylmethyl starch and hydroxypropylated starch.

Hydrophobic polymers (e.g. PLA, PCL or cellulose) in the presence of an appropriate

compatibilizer.

Traditional plastic processing techniques used for TPS modifications are described below.

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22 | P a g e

Extrusion

Extrusion is the most extensively used technique for the processing of starch-based polymers. It

combines numerous unit operations, including mixing, kneading, shearing, heating, cooling,

shaping, and forming. The material is converted to a semi-solid mass using compression and forced

to pass through a regulated opening at a pre-determined rate [6]. The residence time in the extruder

is short (~10 to 60 seconds), while the temperature can be as high as 200ºC [106]. Extrusion

cooking is a high temperature short time process (HTST). For starch modification, it can be used as

chemical reactor, such as thermo-mechanical gelatinization, liquefaction, esterification, and

etherification [6].

The extrusion process can handle high viscosity polymers in the absence of solvents, can be

used at a broad range of processing conditions, it offers the possibility of multiple-injection, and it

allows control of both residence time (distribution) and the degree of mixing [107]. Starch extrusion

has been practiced in the food industry for several decades and has recently gained increasing

attention due to the interest in biodegradable TPS [108, 109]. However, some specific challenges

are present in starch extrusion, including [2]:

Different physical and chemical reactions may occur during processing that can be

favorable and unfavorable.

The rheological behavior of TPS is complex compared to conventional polymers,

especially when mixed with other polymers and additives.

The viscosity of TPS is high under regular extrusion conditions and will in many cases not

lower at increasing process temperature.

The evaporation of water can produce unwanted bubble formation in starch-based

materials.

Several methods of extrusion are available. Few of them are mentioned here.

1. Two stage film/sheet extrusion

2. Extrusion film blowing

3. Multilayer co-extrusion

4. Foaming extrusion

Injection molding

Injection molding is one of the most widely used processing techniques for manufacturing

plastics parts. The desired product is obtained by forcing the molten polymers under pressure into

an evacuated cavity [110]. It is characterized by a high degree of automation, high productivity, and

good dimensional stability of moldings. Injection molding is a cyclic process and consists of four

significant processing stages i.e. filling, packing, cooling and ejection [110, 111]. The process starts

with feeding hot polymer melt to the mold cavity at injection temperature. This is called the “filling

stage”. Next is “post filling stage”, in which the additional molten polymer at high pressure is

packed into the cavity to compensate for the expected shrinkage as the polymer solidifies. This is

followed by the “cooling stage”, where cooling of the mold takes place until the part is satisfactorily

rigid to be ejected. The “ejection stage” is the last step in which the mold is opened and the part is

Introduction and Scope of the thesis

23 | P a g e

ejected [111]. Main factors affecting molding are: materials, molding machine, model design, and

process conditions [110].

Compression molding

Compression molding has been widely applied and examined for the production of starch-based

plastics. This process is investigated for manufacturing foamed containers, and usually consists of

the following three steps; starch gelatinization, expanding and drying. Gelatinization and releasing

the product from the mold are two important steps of compression molding, so gelatinization agents

and mold releasing agents such as magnesium stearate and stearic acid are often used in formulation

to prevent the sticking of starch to the mold [2].

Film casting

Film casting has been widely used and reported in starch literature [105, 112-115]. The process

consist of four main steps i.e. solution preparation, gelatinization, casting and drying. Starch solid

concentrations of 3-10 % and plasticizers are directly mixed in water, transferred quantitatively to a

Brabender viscograph cup, in which the solution is heated from room temperature to 95 °C. The

starch plasticizer mixture is blended for 10 min. The temperature of the Brabender depends on the

type of plasticizer used; for example if glycerol is added to the formulation, a higher temperature

can be used. Immediately after mixing, the gelatinized suspensions are poured onto a flat Teflon or

acrylic plate. Then placed in oven for ~ 24 h at about 40-75 °C and dried till constant weight [2].

Plasticizers help to ease the processing of starch and improve the mechanical properties of TPS

films e.g. the flexibility of films increases by the addition of a suitable plasticizer. Glycerol and

sorbitol are widely used plasticizers to make films flexible. Other chemicals such as sugars, sucrose,

glucose, xylose, fructose, urea and various glycols have been investigated [100, 115, 116].

However, plasticized films lack strength and as a result, blending [25], or laminating [117] with

other materials has been used to overcome this disadvantage. Aqueous blends of cellulose acetate

and soluble starch has been studied intensively [76, 118-120]. Properties of such blends are suitable

for a wide range of biomedical applications, from bone replacement to engineering of tissue

scaffolds, to drug-delivery systems [2].

Spray drying

Spray drying is a well know unit operation that can also be used for the preparation of starch-based

materials. It is a fast drying process that produces amorphous or semi-amorphous particulates from

liquid solution droplets [121]. Pre-gelatinized starch may be produced by spray drying without

losing granular integrity [36, 122]. It is successfully used in pharmaceutical and food technology

[123, 124]. It has been estimated that more than 15000 spray dryers of industrial size are in

operation throughout the world, and approximately double that number is used in pilot plants and

laboratories [123]. It is a flexible process with a lot of possibilities to improve and modify the

characteristics of powders such as particle size, particle morphology, crystallinity and the amount of

residual solvent [125]. It is characterized by high reliability, reproducibility and possible control of

particles size. The spray drying principle is based on the nebulization of a polymer solution

containing the active ingredient as solute or in suspension through a desiccating chamber. A stream

of heated air is used to evaporate the solvent rapidly transforming the small droplets into solid

Chapter 1

24 | P a g e

micro-particles [126]. Nevertheless, spray drying requires particular attention in the process control

because of the high number of parameters. These limitations include problems with efficient particle

collection and degradation of materials sensitive to high temperatures. Each process variable is

critical and requires evaluation of parameters concerning both spray-dryer and feed formulation

[127]. Feed that is to be dried is sprayed in a large cylindrical and usually vertical chamber. A large

volume of hot gas is used to supply sufficient amounts of heat necessary for complete evaporation

of the liquid. Heat and mass transfer takes place by the direct contact of gas and dispersed droplets.

After drying the granular solid and the cooled gas are separated [128]. Classification of spray dryers

can be made on the basis of several parameters, but three of them are important [129, 130].

1. Height-to-diameter ratio of the dryer chamber

2. Air-flow type

3. Atomizer type

Depending on chamber height and diameter tall and short designs are available. A height–to-

diameter ratio of 5:1 is the standard for tall designs versus a 2:1 ratio for short dryers. The latter are

more commonly used [129]. For airflow, three common configurations are used: co-current,

countercurrent and mixed flow, describing the flow directions of air and product through the dryer

chamber. Similarly three types of atomizers are available, rotary or high speed centrifugal disks,

high pressure nozzles and two fluid or pneumatic atomizers [130].

Spray drying for TPS blends

A major drawback of using TPS-based material is their sensitivity to water. Slow

recrystallization of starch (retrogradation) is a major problem for the use of TPS blends for the

manufacturing of packaging and coating materials. The recrystallization is stimulated by the

presence of water and results in the loss of mechanical properties such as flexibility and tensile

properties [104].

In general all the above mentioned TPS processing methods (extrusion, film casting, injection

molding) are performed in the presence of water-plasticizer combinations. The plasticizers used are

mostly hydrophilic. Moreover, the extrusion process cannot be performed without the addition of

water. Combinations of plasticizers and water lead to retrogradation through recombination of

amylose and amylopectin and crystallization is promoted, resulting in the undesired changing of the

final product properties [39].

To overcome this “disturbing” influence of water but maintaining plasticizer activity during the

manufacturing process, another thermal processing route may be needed to obtain “dry” amorphous

starch-plasticizer blends. For this reason spray drying TPS blends from starch water dispersions and

solutions has been investigated. Spray drying processes applied to starch solutions/dispersions for

the preparation of TPS blends is underdeveloped while this technology is considered as an

important way to produce amorphous materials [125] or, in particular for native starches, to

manufacture small amorphous particles [36].

Thesis outline

The objective of this thesis is to study the performance of thermoplastic starch films produced

via compression molding of spray dried starch/plasticizer blends. Through this technique it should

Introduction and Scope of the thesis

25 | P a g e

be possible to obtain moisture-free amorphous starch-plasticizer blends with the potential to be used

as biodegradable bio-plastics. Different plasticizers were used and the properties of TPS blends

were investigated. Special focus is put on retrogradation and the degree of crystallinity of films at

different humidity levels.

Chapter 2 describes the spray drying of different amylose/amylopectin blends for the purpose

of subsequent processing of these materials into TPS films. Spray-drying of starch/maltodextrin

formulations has been evaluated as a potential technology for the manufacture of amorphous

thermoplastic starches (TPS). The properties of solution and dispersion dried TPS samples of maize

starch with different amylose to amylopectin ratios were systematically studied before and after

spray-drying.

Chapter 3 describes an in-depth study of the influence of malto-oligosaccharide molecular

weight, i.e. dextrose equivalents (DE), on the performance of the powders and the films obtained by

compression molding of spray dried powders. Maltodextrins of different molecular weights were

used to evaluate whether they act as processing aid or as plasticizer.

Chapter 4 focuses on the processing of plasticized starch via classic solution casting and by

compression molding of spray dried powders, respectively. Glycerol and urea were used as

plasticizer, separately and in combination with maltodextrins (DE =19.1). The rate of retrogradation

and crystallinity of TPS films produced by both techniques are investigated. As the production

methods are not the same, differences in starch-plasticizer interactions are expected which should

lead to changes in the degree of crystallinity and rate of retrogradation.

Chapter 5 aims to gain increased insight in the retrogradation of plasticized amorphous TPS

films, obtained via compression molding of solution spray dried powder. Starch is plasticized by

using amino acids and malic acid, all having similar molecular weights but different hydrogen

bonding functionality. The samples were also co-plasticized with standard additives like glycerol,

urea, and maltodextrin. Special emphasis was on the retrogradation and mechanical properties of the

resulting films.

In Chapter 6, cross-linked oxidized thermoplastic potato starch (TPS) based films were

prepared through ultra violet (UV) irradiation. The performance of TPS powders and films was

studied. The films were formulated with malic and citric acid as plasticizers in combination with

glycerol as co-plasticizer. Sodium benzoate (SB) was used as photosensitiser. Besides the effect on

retrogradation, the mechanical properties of the films have been studied as a function of UV

treatment and moisture exposure.

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